1. Field of the Invention
The present invention relates to an apparatus for reading an image on a medium to be read, the apparatus including a light emitting device illuminating the medium to be read and a light receiving device receiving reflected light from the medium to be read.
2. Description of the Related Art
A facsimile machine is known as an example of a type of image reading apparatus. The type of apparatus is constructed to include: a light emitting device that emits light for illuminating an original document (i.e., a medium to be read); and an image sensor (functioning as a light receiving device or an image pickup device) that receives reflected light from the original document. Because of the configuration, the apparatus is operated to obtain a plurality of light sensitive signals from a plurality of light receiving elements which are aligned with each other in the image sensor, respectively. The plurality of light receiving elements are respectively associated with a plurality of pixels. The apparatus is further operated to read an image on the original document based upon successive sets of the light sensitive signals obtained from the respective plurality of light receiving elements.
For this type of image reading apparatus, a case exists in which a foreign matter is present in an optical path along which light travels from the light emitting device into the image sensor for light reception, upon reflection at the original document on the way. In this case, the actual amount of light received at at least one of the plurality of light receiving elements which is positioned at a corresponding portion of the optical path to the position of a foreign matter is different from a normal amount of that. It is therefore likely that the apparatus will become incapable of a normal image reading of an image on the original document.
Another case exists in which at least one of a plurality of light emitting elements of the light emitting device or at least one of the plurality of light receiving elements of the image sensor malfunctions. In this case, the malfunction results in an abnormality in obtaining a light sensitive signal from each corresponding one of the plurality of light receiving elements, and it is therefore also likely that the apparatus will become incapable of a normal image reading of an image on the original document.
Japanese Patent Publication No. Hei 08-172530 discloses a conventional technique of detecting an abnormality due to a foreign matter. The conventional technique is practiced such that a white reference member is illuminated with light from a light emitting device, and such that a light sensitive signal is obtained from each corresponding one of a plurality of light receiving elements of an image sensor. Subsequently, a detecting operation is performed for detecting at least one abnormal pixel of a plurality of pixels. The plurality of pixels correspond to the plurality of light receiving elements, respectively, and the at least one abnormal pixel corresponds to at least one of the plurality of light receiving elements which outputs a light sensitive signal indicative of the amount of light received not larger than a predetermined one. During the repeated detecting operations, when the detection of abnormal pixels is continuously repeated a predetermined number of times, it is determined that an optical system, or the like used in the above conventional technique has suffered a persistent abnormality (due to contamination, a foreign matter such as a shading matter, or degradation). As a result, the user is alerted to make the user to realize the fact.